Lift trucks with the remote lift and lower feature installed are also known in the trade as "man-up" vehicles. These vehicles feature an operator station that is mounted on, and travels with, a fork carriage supported by an extendable mast. The extendable mast and fork carriage raises and lowers the operator's station. The operator will often stand on a platform attached to the forks and operate the remote lift and lower controls. The controls are initiated by turning a key switch to activate the remote mode. The remote mode of operation restricts all other functions, such as vehicular travel, fork reach, fork retract, fork sideshift and the fork tilt. This configuration uses a controlled lift/lower speed function. The speed is predetermined by the setting of a hydraulic flow-control valve that regulates the fluid supplied to or released from the lift cylinders.
During the controlled lowering of the lift forks, the above system will experience a drop in hydraulic pressure between the lift cylinders and the flow control valve when an obstruction prevents lowering. This drop in hydraulic pressure causes the lift chains on the cylinder(s) to become slack. The fork-lowering procedure can then become erratic and non-uniform if the obstruction is suddenly removed.
Recent lift truck designs have included a pressure transducer in the hydraulic circuitry to monitor the weight being supported upon the forks. This pressure transducer also determines whether to utilize a second stage hydraulic pump in a tandem pump system if a preset pressure has not been exceeded.
The present invention provides a method to utilize this pressure transducer for monitoring the hydraulic pressure during the remote mode, i.e., when the operator's platform is being lowered. The invention turns off the normally closed load-holding value when the pressure drops below a predetermined value, which stops the flow of hydraulic fluid back into the reservoir. This, in turn, stops the cylinder(s) from lowering, and keeps the fork carriage lift chains from becoming slack.